This invention relates generally to a method and apparatus for composting source material, and more specifically to a method and apparatus for maintaining composting conditions within preselected ranges.
While landfill space is dwindling or becoming more costly or distant, industrial and municipal waste production is increasing. Furthermore, an ever-increasing variety of materials are developed and introduced into the waste stream. Consequently, the safe disposal of solids resulting from wastewater treatment is continually more challenging and complex.
Composting, which, for the purposes of this disclosure will be defined as xe2x80x9cthe use of living, microbial organisms to convert solids and liquids into more environmentally safe and/or beneficial by-products,xe2x80x9d is a potentially viable alternative to landfills for the disposal of organic materials from nearly all waste streams. Composting can be used, for example, to process municipal wastewater biosolids, to remediate industrial waste water solids, and to treat wastes and by-products from processing food and agricultural products. Composting can reduce the volume of organic waste materials by 50% or more, yielding a stable, non-odorous material that can be used as fertilizer or as an amendment for soil.
Before about 1970, composting was typically a simple process in which waste materials were piled and allowed to sit until they decomposed. It was most frequently done on a small scale and was not often considered for industrial-scale problems. Grinding the material to be composted was considered advanced technology.
An advance in composting technology came from the realization that adding air to the composting mixture could increase the efficiency of composting. The microbes that produce compost require air and will smother inside of a static unaerated pile. Hence, the initial methods of aeration involved moving or agitating the compost to allow air into the stack.
A typical example of this aeration is a windrow turner that picks up the compost and dumps it to one side. Approximately 95% of municipal composting sites are currently windrow turner operations, though process control is, unfortunately, quite primitive. Piles are typically turned at the convenience of the operator, rather than to optimize the composting process. A typical pile of compost will use all of its oxygen within about one-half hour, so such windrow turning is seldom related to actual oxygen demand. Turning is done seldom enough that microbes in the center of the pile are rapidly depleted, and the center of the pile stops composting. Turning the pile merely re-inoculates the center material with fresh microbes, and composting continues in the center of the pile for another one-half hour when the oxygen supply is, once again, depleted. Unfortunately, the repeated mechanical actions that are required for turning destroy some beneficial fungi that rely on large, filamentous growth. In addition to the oxygen and mechanical problems introduced by a windrow system, composting with windrow turners is typically done in an open, unsheltered area. The vagaries of weather and rainfall most often determine the water content of the composting mass. When there is too little rain, the pile is too dry. When there is too much rain, the pile is wet and requires frequent turning. Too much rain can also lead to problems with runoff of leachate.
One method used to overcome some of the disadvantages of pile composting is to enclose compost piles in a building. An enclosure that keeps rain off of the compost allows better regulation of water content. However, such a facility is very expensive. Furthermore, with pile composting, various irritating and potentially toxic gases are sometimes produced. Since operators must enter the enclosure to maintain the composting process, enclosing compost also involves maintaining the quality of large volumes of air within the building. Without high-quality and high-quantity air handling systems, the atmosphere within an enclosure can be irritating, if not toxic, to an operator.
Some of the disadvantages of pile composting are overcome by more modern reactor vessel processes. By design, the reactor vessel is typically only slightly larger than the compost which it contains. This reduces the land area required to store the compost during the composting process. In addition to reduced land area, the total volume containing or enclosing the compost is also reduced. Lower total volume means reduced air handling requirements. Furthermore, in-vessel reactors also provide the opportunity for collection of potentially odorous emissions. The compost is enclosed, and exhaust air may be routed through a filtration system. This separation of operator from compost air benefits the health and safety of all operators. There are other benefits, beyond land space and air handling, from reactor vessels. Handling and mixing, which is required in all systems, can also be mechanized using reactor vessels, and the compost is enclosed.
Unfortunately, vessel systems to date are complicated systems which require precision construction techniques and permanent, stable foundations. This necessarily drives the cost of present reactor vessels systems to levels even higher than required for building-type enclosures. In exemplary prior art systems, organic waste is fed into an opening at one end of the reactor and compost is removed from the other end. The material is moved through the reactor by, for example, a complex moving floor apparatus or hydraulic ram. Aeration is sometimes provided by pressurized air forced through the organic waste from air vents located throughout the moving apparatus.
Some in-vessel systems also include mixing systems, typically rotating paddles or prongs, within the compost mass. Other in-vessel systems are static. The agitation systems used with in-vessel systems are expensive, prone to wear and failure, and provide agitation at intervals that are not readily controlled with respect to the progress of the composting process.
There remains a need for an improved and economical in-vessel composting process which is convenient, low-cost, efficient, odorless, scalable, and not labor intensive.
The present invention provides a method of composting large quantities of organic material under controlled, preselected conditions. Conditions which are known and documented elsewhere as being beneficial to composting, such as temperature, water level, carbon and nitrogen content, oxygen supply, microbial activity, and other similar conditions may be monitored and adjusted through the preferred method and apparatus of the invention. In addition, the operator is not exposed to hazardous gases with the present apparatus. As a result, the preferred method and apparatus allow an operator to more safely maintain both the composition of the material and the processes within preselected, optimal limits. Through careful selection of available components, the inventive apparatus may be manufactured at costs that are a fraction of those previously available for vessel-type reactor systems, using only a fraction of the real estate previously required, while obtaining enhanced results.
The invention includes a method for composting source material within a sealable container. Adjusting the composition of the source material to within preselected limits converts the source material to a compostable mixture. The composition is adjusted by amending the source material and by mixing the amended material with a bulking agent and an inoculant. The method includes the steps of monitoring and adjusting conditions of the composting mixture to maintain conditions within preselected limits. The conditions of the composting mixture are adjusted by aerating the composting mixture and/or re-mixing the composting mixture. The remixed composting mixture is incubated in order to yield composted organic material. Preferably, re-mixing includes transporting the composting mixture from the site at which it is incubated to a tipper and/or mixer separate from the incubation site.
The invention also includes an apparatus for composting source material. At least one sealable container converts a source material to a composted material. An air management apparatus delivers air to the composting mixture, a control unit measures temperature within the composting mixture, and a biofilter removes odors from the air to prevent escape of noxious odors from the system. The sealable container can be filled with source material, and then the material can be dumped from the sealable container, which simultaneously cleans air vents in the container floor. The partially composted material can be amended and re-mixed, and then the material can be returned to the sealable container. Also included in the system is a dumping system for removing composting material from the sealable container. The system can optionally include at least one curing bin for storing the composted material, after the composting process, in a sealable container.
The system of the invention can be tailored to the individual needs and limitations of the site where the system is located. For example, a system for highly populated urban areas can comprise a sealable container that can hold approximately 1 cubic yard and a dumping system that might comprise a small forklift. A mid-sized system can comprise a plurality of 40 or 50 cubic yard roll-off garbage containers and a tippable roll-off truck. A large system can comprise a plurality of 80 cubic yard or larger intermodal containers located at a shipping port or railroad yard, a dumping system and an overhead crane. The system includes an apparatus for transporting the sealable container from the site at which composting occurs to a mixing station, where re-mixing occurs, and/or an apparatus for dumping or tipping the container at the mixing station.